The present invention relates to a passivation process and structure with self-alignment with the location of a mask. More particularly, the present invention relates to a self-aligning process for a passivation layer chemically deposited in the vapor phase on the location of a diffusion mask.
During the manufacture of a planar-type device, it is necessary during certain steps to carry out diffusions in a semiconductor wafer through selected openings provided in a mask formed on the semiconductor wafer, this mask being for example formed from a thermal or pyrolytic oxide layer. The diffusion penetrates into the semiconductor wafer and extends transversely under the masking layer. This masking layer may be maintained during further manufacturing steps to serve as passivation for the surface flush portion of the junction. Nevertheless, in some cases, the leak current in the vicinity of the flush portion of the junction may be too high with such a passivation layer. It is then preferable to replace the masking layer with a layer or an assembly of layers of other passivation materials deposited chemically in the vapor phase. There will be more particularly considered here the case where it is desired to use as passivation agent polycrystalline silicon doped with oxygen, called hereafter sipox, but the invention also applies to the case of other chemical deposits in the vapor phase, for example silicon nitride or silica, doped or not.
To better illustrate the disadvantages of the process of the prior art, there will first of all be described hereafter in detail with reference to FIGS. 1a to 1g a process of the prior art for replacing an oxide mask with a sipox layer.
We consider initially a semiconductor structure comprising a type n layer 1 covered on its lower face with a type n.sup.+ layer 2 and in the upper face of which is diffused a p.sup.+ type layer 3. This localized layer 3 is obtained by diffusion through a masking layer 4, formed for example by an oxide. The diffusion process selected is such that there is produced, above diffused zone 3, a thin oxide layer 5.
In the following FIG. 1b there is illustrated the depositing of a photosensitive resin layer 6 above the thinner part 5 of the oxide layer, i.e. above the openings in the masking layer 4.
FIG. 1c illustrates the condition of the structure after etching of the silica layer 4 and removal of the photosensitive resin layer 6.
In a further step, illustrated in FIG. 1d, a first layerof polycrystalline silicon doped with oxygen (sipos) 7 is deposited on both faces of the device, followed by a second silicon nitride layer 8.
In the step of FIG. 1e, a resin layer 9 is deposited on the upper face of the wafer and opened in a region corresponding substantially to the openings of the first mask 4. After chemical etching, the structure shown in FIG. 1f is obtained.
In the following step, illustrated in FIG. 1g, the visible part of silica layer 5 is removed by chemical etching. It will be noted that there remain lateral portions 5' and 5" of this oxide layer 5 enclosed under the passivation structure comprising the sipos layer 7 and the silicon nitride layer 8.
Thus, in the process of the prior art, it will be noted that two maskings are provided: the first for forming the localized resin layer 6, of FIG. 1b, the second for forming the localized opening in the resin layer 9. It is because of these two maskings that a certain tolerance must be provided in the dimensions of the masks so as to be certain that the layer of sipos finally obtained will sufficiently cover type p.sup.+ zone 3.
An aim of the present invention is to provide a new process for depositing a passivation structure whose basic layer is deposited chemically in the vapor phase, this layer being self-aligned with an initial masking layer through the openings of which localized diffusion has been formed in a semiconductor wafer.
Another aim of the present invention is to provide such a process which is simple to use industrially.
Another aim of the present invention is to provide such a process which lends itself particularly simply to deposition of a metallization layer in the opening provided in the passivation structure.